Where would YouTube be without reaction videos? Kids’ reactions, moms’ reactions, reactions to scary pranks or beautiful proposals. A genre that was once non-existent has grown to become one of the most popular forms of video on the platform, bringing people back for “lols” and “awws” time and time again. In fact, one of YouTube’s earliest viral hits was a reaction video -- the original Scary Maze Prank, which is currently sitting pretty at 27 million views.

Reaction vids have been a YouTube staple for years. But it wasn’t until The Fine Bros React series that the idea hit new heights. Debuting just a few weeks before the fifth anniversary of YouTube’s official public launch, the series began with kids simply reacting to popular YouTube videos. It was a meta moment that proved an instant hit, pulling in more than 10 million views to date. Five years later, the Bros have garnered more than 12 million subscribers and nearly 3 billion views across their 800+ videos.

GOTCHA. Because how could we talk about the “R” of the YouTube alphabet without a Rick Roll?! A cultural phenomenon that took off in 2007, Rick Astley’s “Never Gonna Give You Up” still sees a spike every April Fools’ Day, with friends performing this musical bait-and-switch all over the world. In fact, the largest spike in views in the video’s history happened just this past April Fools' Day when the video was viewed over 2 million times. Proof that the Rick Roll is still a sure-fire way to get a reaction out of anyone. See what we did there?